The epidemiology of pertussis in England and Wales.

Recovery in the uptake of whole-cell pertussis vaccine from 30% in 1978 to 91% in 1992 has resulted in a major reduction in the incidence of pertussis and in the proportion of cases occurring in pre-school children. This has been accompanied by a decline in pertussis mortality rates, with no deaths reported since September 1990. Seventy-four per cent of the 50 fatal cases reported since 1980 have been infants under one year of age, demonstrating the importance of completing primary immunisation as early as possible after birth. There is no evidence, as yet, that introduction of the accelerated immunisation schedule in June 1990 has reduced the proportion of cases occurring in infants, suggesting that the decline in mortality during the last two years is largely due to indirect protection provided by the current high level of vaccine-induced herd immunity. If 90% coverage is sustained and there is no significant waning of immunity with age, the circulation of Bordetella pertussis should be sufficiently reduced to interrupt the four year epidemic cycle for the first time in the United Kingdom. The hypothesis that vaccine-induced immunity has a serotype-specific component is supported by recent changes in serotype prevalence which have followed the increase in vaccine coverage. Given the successful reinstatement of whole-cell vaccine in the UK immunisation programme, the case for replacing it with a new acellular preparation will require careful evaluation.